1. Field of the Invention
The present invention relates to a commutator of a small-sized DC motor that performs electrical opening and closing by mechanical sliding motions and, more particularly, to a commutator for a small-sized DC motor used in the loading and unloading of a CD in a CD player and in the pick feed for moving a lens to read out CD signals, and a material for sliding contacts that constitutes a commutator (in addition, an earth ring, a rotary switch, etc.) of a small-sized DC motor used in home electric appliances that are driven by charging type batteries.
2. Description of the Related Art
In recent years, in the above-described technical field, research on new materials for sliding contacts has been actively carried out. With respect to a material for sliding contacts that constitutes a commutator of a small-sized DC motor, it can be said that it is the most important development problem to make the wear during the use of contacts ideal and to realize low contact resistance. Essentially, low contact resistance of a material for sliding contacts can be realize by ensuring that the materials that should come into contact with each other come into positive contact or are brought into close contact, to say nothing of the electrical conductivity of the material for sliding contacts itself. However, when the material slides, the higher the degree of contact or close contact of the materials that come into contact with each other, the higher the frictional resistance will be. And if sliding is caused against this friction, remarkable wear phenomena will occur. That is, in a material for sliding contacts, it is impossible to obtain those having ideal properties unless the above-described phenomena that mutually contradictory are essentially controlled. Also, there are many scientifically unclear points in the wear phenomena of sliding contacts and it is said that to control the wear phenomena by improving materials for sliding contacts is very difficult.
Types of wear in materials for sliding contacts are broadly divided into adhesive wear and abrasive wear. Usually, even when the surface of a material for sliding contacts is finished considerably smooth, microscopically the surface is not a complete plane and many fine irregularities are present. When such metal surfaces are brought into contact with each other, apparently they seem to be in contact with each other with a wide area. In reality, however, protuberant portions of the fine irregularities present on the surfaces are in contact with each other, and hence what is called a true contact area is small than an apparent contact area. For this reason, a large pressure is applied to this true contact part, i.e., the protuberances that have come into contact and the coagulation of metals that come into contact with each other occurs, as a result of which a soft metal is torn and transfers into a hard metal. This is coagulant wear. Also when materials having different hardness come into contact with each other or when in the case of contact of soft metals, one soft metal contains hard particles, the soft metal is mechanically sheared by the hard metal and scratch wear occurs.
These wear phenomena depend greatly on the hardness of metal materials that come into contact with each other, bonding properties of these metals, etc. And basically, the wear phenomena of materials for sliding contacts become remarkable in proportion to contact pressure and are reduced by the hardening of materials. However, wear phenomena change remarkably depending on changes in temperature and humidity during contact and the presence of corrosive components, organic steam, dust, etc. And because these changes in wear phenomena are changes in the contact condition in contact points, they cause an increase in contact resistance and have a great effect on stable keeping of low contact resistance.
When a clad composite material that uses a material for sliding contacts is incorporated in a small-sized DC motor as a commutator and the motor is driven at high speed revolutions, the above-described wear phenomena occur between the commutator and a brush. That is, the material for sliding contacts that constitutes the commutator is subjected to contact friction for a long time and frictional heat by sliding is added, with the result that the above-described coagulant wear and scratch wear occur in a complex manner. For this reason, the surface of the material for sliding contacts is ground by the wear phenomena and wear particles occur. The wear particles increase contact resistance and fill gaps of the commutator, causing conduction and short-circuiting, generating noise and the like.
Furthermore, when the wear phenomena proceed, in a clad composite material that uses a material for sliding contacts, a metal provided in the surface layer of the clad composite material, i.e., the material for sliding contacts is broken by wear and it follows that the wear proceeds to a base material under the surface layer. When the base material of this clad composite material comes to a state in which the base material is worn, the base material that is apt to be oxidized becomes exposed and, therefore, various electrical troubles may sometimes be caused by metal oxides of the base material. For this reason, when what is called a two-layer or three-layer clad composite material is formed and used as a commutator, it can be said that improving the material for the alloys that compose each layer is a very important problem.
In recent years, as materials for sliding contacts for a commutator for a small-sized DC motor used in the loading and unloading of a CD in a CD player and in the pick feed for moving a lens to read out CD signals and materials for sliding contacts for a commutator of a small-sized DC motor used in home electric appliances that are driven by charging type batteries, there have been used two-layer clad composite materials in which an Ag—Cd alloy containing 1 to 2% Cd by weight and the balance Ag is used as the surface layer and Cu or a Cu alloy is used in the base layer (for example, Ag99-Cd1/Cu), two-layer clad composite materials in which an Ag—Cd—Ni alloy containing 1 to 2% Cd by weight, 0.01 to 0.70% Ni by weight and the balance Ag is used as the surface layer and Cu or a Cu alloy is used in the base layer (for example, Ag97.7-Cd2-Ni0.3/Cu), etc. The “alloy composition/Cu” described in parentheses means a clad composite material that constitutes two layers and “/” means an interface between the surface layer and the base layer. The numerals described behind the alloy composition elements indicate values in % by weight.
These Ag—Cd alloys and Ag—Cd—Ni alloys are materials excellent in electrical properties, hardness and low contact resistance and they are disclosed, for example, in the Japanese Patent Publication No. 2-60745 as a material for sliding contacts for a commutator of a small-sized DC motor that is an Ag alloy containing 1 to 5%, by weight, of at least one kind selected from the group consisting of Sn and Cd and the balance Ag. However, when the environmental problems of today and the like are considered, the manufacturing and use of materials for sliding contacts that contain Cd, which is considered a harmful substance, are not desirable.
Ag—Cu alloys, Ag—Cu—Cd alloys, etc. are also used as other alloy systems. However, in these materials for sliding contacts, changes with time occur in contact resistance although the contact resistance in the initial stage of use is low. For this reason, this poses the problem that a deterioration in the product value of shavers using charging type batteries occurs. That is, when materials for sliding contacts of these alloy systems are used in a small-sized DC motor are used, the start voltage of the motor increases because contact resistance increases due to changes with time. In other words, the time in which the electromotive force of the battery decreases to below the start voltage of the motor becomes short, and this poses that problem that the motor does no start immediately. As a result, the charging frequency of the battery increases and the life of the battery itself shows a tendency to become short.
Also, for example, the Japanese Patent Laid-Open No. 58-104140 discloses a material for sliding contacts of Ag—Zn-based alloy in which 1 to 10% Zn by weight and 0.5 to 1.0%, by weight, of at least one kind selected from the group consisting of Te, Co, Ni, Cu, Ge, Ti and Pb are added to Ag. In this material for sliding contacts, by making the most of the nature of Te, Co, Ni, Cu, Ge, Ti and Pb that these metals are more easily oxidized than Zn, these metals are contained, whereby the oxidation of Zn is suppressed, the sulfuration resistance and lubricity of the material for sliding contacts are maintained, and the improvement of wear resistance and stabilization of low contact resistance are aimed at. However, also in this material for sliding contacts, as with the above-described Ag—Cu alloys and the like, changes with time occur in contact resistance tends to increase when the use period becomes long although the contact resistance in the initial stage of use is low.
Furthermore, the Japanese Patent Laid-Open No. 8-260078 discloses materials for sliding contacts of Ag—Zn alloy and Ag—Zn—Ni alloy. Also these materials for sliding contacts have low contact resistance, they cannot be said to be materials for sliding contacts that can control the wear phenomena to such an extent that motor life can be improved.